Joint cartilage is fibrous connective tissue providing cushioning between bones. Disease and physical disorders, such as osteoarthritis, rheumatoid arthritis, and avascular necrosis; benign or malignant bone tumors; excessive physical activity; and hereditary defects can cause cartilage tearing, erosion, or degeneration. Osteoarthritis, for instance, can occur following trauma to a joint region, or due to genetic predisposition or obesity. The disease is characterized by a premature wearing down or “erosion” of the cartilage surrounding a joint between two bones. The wear can lead to the bones rubbing directly against one another, which in turn causes pitting and malformation of the bone surfaces accompanied by pain and encumbrance of range of motion. Osteoarthritis treatment regimens include resting the affected joint, prescription of pain relief and anti-inflammatory medication, improved diet, and low impact exercise. In severe cases, surgical intervention, such as arthroplasty surgical procedures, may be necessary to repair the damaged or dysfunctional joint surfaces.
Hip joints are particularly susceptible to cartilage compromise and hip arthroplasty, commonly called “total hip replacement” (THR), attempts to relieve the pain associated with, and to restore the integrity and functionality of, damaged hip joints. In THR, the upper portion of the femur, including the femoral head and neck, is removed to receive the stem portion of a prosthetic implant. While generally successful, further post-THR hip joint deterioration may necessitate revision surgery, which entails radical femur restructuring by splitting apart the femur to remove the THR prosthesis stem and surgically rebuilding the femur, a more costly and involved procedure with extended convalescence.
Recently, hip resurfacing has emerged as a viable surgical alternative to THR, which is especially suitable for younger and more active patients. Hip resurfacing entails implantation of a hip joint prosthesis, generally formed of a femoral head prosthesis and an acetabular prosthesis. Unlike THR, the upper portion of the femur is retained intact and the femoral head is, instead reshaped to accept a less extensive prosthetic femoral cap. Resurfacing requires less bone removal, which can result in easier revision surgery, if later needed, by preserving more bone stock.
Precise alignment of the femoral head prosthesis along the central access of the femoral neck and of the acetabular prosthesis to the acetabulum is essential to successful hip resurfacing. Any misalignment can result in pain and affect the degree of leg extension and joint rotation. Typically, X-rays, computed tomography (CT) scans, magnetic resonance imaging (MRI), or other forms of non-invasive imaging are taken of the hip joint and surrounding bony structures for planning the hip resurfacing procedure. Surgical templates are used along with the images to approximate the size and implantation axis of the prosthesis.
The preoperatively planned positions of the femoral head prosthesis and acetabular prosthesis must be transferred to the actual patient in the surgical theater, which is a manual and inherently imprecise process. During conventional hip resurfacing surgery, the acetabulum is resurfaced along a preoperatively planned acetabular prosthesis implantation axis. Determining the position and axis of the acetabular prostheses during surgery, even with preoperative planning, is a manual, lengthy, and potentially error prone procedure, highly dependent upon the skill of the surgeon, quality of imaging, and patient condition.
Conventionally, the implantation axis of the acetabular prosthesis is manually approximated using a reaming device prior to proceeding with the acetabular resurfacing surgery. Alternatively, a guide wire can be inserted into a nearby bone region of the patient along the approximated implantation axis. With practice, an acceptable guide wire position can be determined, yet correct placement requires frequent manual readjustments of the guide wire before proceeding with reaming of the acetabulum.